Project Summary: The objective of this study is to elucidate the mechanism by which testosterone inhibits spermatogonial differentiation after toxicant exposure of rats. Radiation will be used as a model toxicant in most studies and the relevance to chemical toxicants will be assessed by comparison with results obtained after dibromochloropropane (DBCP) exposure. In particular, the extension to DBCP of the conclusion that radiation induces the spermatogonial differentiation block by damaging the somatic environment, not the spermatogonia, will be examined. In irradiated rats, the source of the factors and the specific factors that are regulated by testosterone to affect spermatogonial differentiation will be identified. Previous results indicated that factors present or transmitted through the interstitium are important. That hypothesis will be tested by examining spermatogonial differentiation in vivo after testicular somatic cell or tubule transplantations or after selective depletion of the interstitial Leydig cells or macrophages, which produce paracrine factors, and by assessing the effects of interstitial fluid on spermatogonial differentiation in vitro. Next, genes coding for growth and differentiation factors that are regulated by testosterone and FSH in a manner coordinate with spermatogonial differentiation will be further tested for their correlation with spermatogonial differentiation using other hormonal modulations;the gene that best correlate will be studied, along with their receptors, in depth. An in vitro system will be used to determine whether testosterone or other hormones directly modulate spermatogonial differentiation independent of systemic effects that occur in vivo and also to test whether the specific protein factors regulated by testosterone affect spermatogonial differentiation. Relevance: An increasing number of men have low sperm counts, possibly due to exposure to known and unknown environmental agents. We have developed a model in which radiation and chemical toxicants produce prolonged reduction or absence of sperm in rodents, despite the presence of spermatogonial stem cells. We have also developed hormonal methods for the reversal of this block in spermatogenesis;however the mechanisms of the block and its reversal are unknown. It is essential to elucidate these mechanisms in order to determine how methods for reversal of that block could be applied to man.